Thickness dependence of spin-orbit torques generated by WTe2

Abstract

We study current-induced torques in WTe2/permalloy bilayers as a function of WTe2 thickness. We measure the torques using both second-harmonic Hall and spin-torque ferromagnetic resonance measurements for samples with WTe2 thicknesses that span from 16 nm down to a single monolayer. We confirm the existence of an out-of-plane antidamping torque, and show directly that the sign of this torque component is reversed across a monolayer step in the WTe2. The magnitude of the out-of-plane antidamping torque depends only weakly on WTe2 thickness, such that even a singlemonolayer WTe2 device provides a strong torque that is comparable to much thicker samples. In contrast, the out-of-plane field-like torque has a significant dependence on the WTe2 thickness. We demonstrate that this field-like component originates predominantly from the Oersted field, thereby correcting a previous inference drawn by our group based on a more limited set of samples.arXiv:1707.03757v1 [cond-mat.mes-hall]

Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide β−MoTe2

“…We note, however, that the relative insensitivity of σ S to the inplane current direction observed in β-MoTe 2 and WTe 2 is not required by symmetry, and in general the magnitudes of the in-plane spins generated in response to a current along the a or b-axes are allowed to differ [34]. We obtain an average value of σ S for our MoTe 2 /Py devices of 5800 ± 160 /(2e) (Ω −1 m −1 ), smaller than the average value observed in our WTe 2 /Py heterostructures, 8000 ± 200 /(2e) (Ω −1 m −1 ) [12,13], and larger than the ≈ 3000 /(2e)…”

“…When a charge current is applied to a material with large spin-orbit coupling, such as a heavy metal [2][3][4][5][6][7], topological insulator [8,9], or transition metal dichalcogenide (TMD) [10-16], a spin current generated through mechanisms such as the spin Hall or Rashba-Edelstein effects can be used to exert a torque on an adjacent ferromagnet. Recent work from several research groups has focused on understanding how a controlled breaking of symmetry in a spin-generating material / ferromagnet heterostructure can be used to tune the direction of the observed spin-orbit torques for optimal switching of magnetic devices [12][13][14][17][18][19][20][21][22][23][24]. For instance, the presence of magnetic order within a spin-generation layer can allow current-generated spin directions that are typically forbidden for highly-symmetric non-magnetic metals [17][18][19][20].…”

“…For instance, the presence of magnetic order within a spin-generation layer can allow current-generated spin directions that are typically forbidden for highly-symmetric non-magnetic metals [17][18][19][20]. Similarly, our group has shown that by using WTe 2 as the spin-source material, a TMD with a low-symmetry crystal structure, it is possible to generate an out-of-plane antidamping torque [12,13] -the component of torque required for the most efficient mode of switching for magnets with perpendicular magnetic anisotropy, but forbidden in higher-symmetry materials. Only one other material, SrRuO 3 , has been shown to generate an out-of-plane antidamping spin-orbit torque [20], arising from symmetry breaking associated with magnetic order.…”

Large Multidirectional Spin-to-Charge Conversion in Low-Symmetry Semimetal MoTe2 at Room Temperature

“…In our experimental geometry, both the conventional and unconventional SCCs can originate from two different mechanisms, the spin Hall effect at the bulk and the Edelstein effect at the surface states. An unconventional charge-to-spin conversion effect has been reported in the low-symmetry 1Td phase of WTe2 15,16 and 1T´ phase of MoTe2 43 using spin-orbit torque measurements, where out-ofplane damping-like torque, in which the spin current is parallel to the out-of-plane spin polarization ( ∥ ∥ ), is observed. Such torque is allowed in crystals with a single mirror plane.…”

Current-Induced Torques with Dresselhaus Symmetry Due to Resistance Anisotropy in 2D Materials

“…That is, the generated field or spin is perpendicular to the applied current and lies within the sample plane. Torques corresponding to a more general spin symmetry have been observed only in non-centrosymmetric systems, such as torques resulting from the out-of-plane spins in WTe 2 , 10,11 or torques corresponding to a Dresselhauslike spin polarization ( Fig. 1b) observed in GaMnAs, 12,13 GaAs/Fe heterostructures 14,15 and NiMnSb.…”